This invention utilizes principles of Josephson junctions and reference may be had in this regard to the article by C. E. Gough entitled Granular Josephson and quantum interference effects in HTC ceramic superconductors, IBM Journal of Research and Development, Vol. 33, No. 3, pp. 262 ff, May 1989; the article entitled High-Temperature Superconductors on the Road to Applications, Nov. 27, 1989, Chemical and Engineering News, pages 9 ff; the article by Alan M. Wolsky et al entitled The New Superconductors: Prospects for Applications, Scientific American, February 1989, pages 61 ff.
In sensors for the measurement of magnetic flux comprising a superconductive ring having at least one Josephson element incorporated therein (see p. 263 of the aforementioned IBM Journal article) and coupled to an electrical oscillatory circuit, i.e. a tank circuit, the superconductive ring having the Josephson element or junction incorporated therein, is fabricated by thin film technology whereby both the ring and the Josephson element may be formed as a thin layer on a substrate by evaporative deposition or cathodic sputtering.
In operation of such sensors, they are generally coupled in an electrical parallel oscillating or resonant circuit, generally referred to as a tank circuit hereinafter, which likewise can be superconductive and can have a resonant frequency which will be designated as f hereinafter.
Impressed upon this circuit is a high-frequency current with the identical frequency f. The superconductive ring acts as a damper for the oscillations of the tank circuit such that the degree of damping is a function of the magnetic flux through the superconductive ring. As a consequence, there is also a change in the voltage drop across the tank circuit which can be read out by appropriate evaluating or detecting electronic circuitry. The circuitry need be responsive, therefore, only to this voltage drop.
It is also known that the intrinsic or characteristic noise of such sensors and, consequently, their maximum sensitivity, can be increased when the operating frequency of the tank circuit is increased.
At the higher frequencies (f&gt;500 MHz) it is difficult to realize such tank circuits from discrete elements like coils and condensers. Furthermore the coupling between the tank circuit and the superconductive ring drops because of the fact that the inductivity of the tank circuit becomes significantly less with increasing frequency, eventually reaching a minimum level at which coupling fails as a practical matter altogether.